What is the time that has to lapse between filing Chapter 13 and Chapter 7?
Generally speaking, there is no bar to filing a Chapter 7 after a Chapter 13. Exceptions to this rule are when (1) prior case was dismissed due to willful failure to abide by orders of the court and (2) voluntary dismissal following the filing of a request for relief from stay. In these cases, there is a 6 month bar.
Your question really has more to do with "discharge" as opposed to "filing" bars. There is a difference between the two. A person cannot obtain a discharge in a Chapter 7 for six years after obtaining a discharge in a Chapter 13, unless the person in the Chapter 13 paid 100% of the unsecured creditors or at least 70% and it is found that the plan was filed in good faith and was the person's best efforts.
This response is not intended to create an attorney-client relationship. It is intended to provide general comment to the question. If you'd like to consult, please see my biography for contact info.